The invention relates to a twin-wire former for producing a fibrous web, more particularly a paper web, and particularly to the support surface, rolls and guide means for the two wires. The invention furthermore relates to a method of manufacturing a fibrous web by means of such a twin-wire former.
A twin-wire former under the name "Bel Baie III" is already known (, Preprints "B", 74th Annual Meeting, Technical Section, Canadian Pulp and Paper Association, p. B286-B289). It has essentially the following construction. Two breast rolls guide two wire screens, called wires, from bottom to top in a substantially vertical twin-wire zone. The two wires converge to define a nip in the vicinity of a curved stationary support device which is arranged directly above one of the two breast rolls. A head box arranged below the breast rolls delivers a jet of fibrous pulp stock into the intake gap or nip formed by the two wires. A fibrous web is then formed from the jet of stock in the twin-wire zone. This is achieved in known manner by the removal of most of the water from the stock, mainly as a result of the tension of the wires and the curved course of the twin-wire zone. The curved wire guide surface of the stationary support device has openings for the removal of water, and at least some of the openings may be connected to a source of vacuum. Following the curved support device in the direction of travel of the wire, there is a forming roll, which is developed as a suction roll. The forming roll and the curved support device are located within the loop of the same wire. For removing the water which has penetrated through the meshes in the outer wire in the region of the curved support device, at least one deflector is arranged in the loop of the other wire.
The known arrangement has the following features. The forming roll and the stationary support device are located in the loop of the second wire. Furthermore, the two wires separate in the upper region of the periphery of the forming roll. As seen in the direction of web travel, the separation occurs at the run-off point of the first wire, which is located in front or upstream of the run-off point of the second wire. The second wire then travels, together with the fibrous web that has been formed, over a suction couch roll where additional water removal takes place and then travels to the customary take-off point.
The known arrangement tries to obtain the following result. The use of a stationary support device of very large radius of curvature arranged at the start of the twin-wire zone is intended to form the fibrous web as gently as possible in spite of a high speed of operation. As a result of the simultaneous removal of water to both sides of the web through both wires, the web to be formed, which is preferably a paper web, is to be imparted with properties which are as similar as possible on both sides, i.e., there is to be little two-sidedness. At the same time, the quality of the paper produced is to be improved in that upon the removal of the water, as little fibrous material and fillers are lost as possible, i.e., there is the highest possible retention.
However, problems arise in this known twin-wire former because the "second" wire comes into contact on the wrapping zone of the forming roll directly with the suction roll shell of the forming roll, which is perforated in the customary manner. This creates a danger that the perforations of the suction roll shell produce a so-called hole shadow marking in the web of paper. This reduces the quality of the finished web of paper. This danger could be reduced or avoided by forcing the water removal and the web formation in the starting region of the twin-wire zone and therefore in front or upstream of the forming roll so that the formation of the web is at least substantially concluded when the forming roll is finally reached. However, such forcing of the water removal at the stationary support device would entail the danger of reduced retention and furthermore could cause the occurrence of so-called needle holes in the web of paper, particularly in the case of relatively thin papers. Such needle holes are presumed to result from the speedy removal of the water being greater than average at individual points of the web of paper.
The known twin-wire zone is curved in only one direction, i.e., there is no countercurvature such as appears, for example, in the S- shaped twin-wire zone of FIG. 2 of U.S. Pat. No. 3,876,499. This presents a danger that there may be a certain two-sidedness of the finished web of paper, i.e. the finished web of paper may not be imparted the same properties to the desired extent on both sides.
Another disadvantage of the known twin-wire former is that the separation of the two webs takes place already in front or upstream of the suction couch roll and therefore occurs at a place where the web of paper formed still has a relatively low content of solids. As a result, a relatively large number of fibers are torn out of the paper web just formed by the second wire upon the separation from the paper web while the web is still being carried further with the first wire. This again impairs the quality of the finished paper web.